Behavioral studies of aging monkeys demonstrate significant impairments that are likely to reflect neuronal dysfunction in the prefrontal cortex and medial temporal lobe but appear to be due to subtle disruption of neuronal function. The mechanisms of this dysfunction will be investigated according to four aims in using physiological, molecular, neuroimaging and neuroanatomical methods. First we will use the in vitro slice preparation to identify alterations in neuronal physiology including mechanisms underlying age-related defects in action potential generation and synaptic function. Second, we will harvest single physiologically characterized neurons from these slices and,in collaboration with Project 2, will use an adapation of single cell PCR to assess the expression of related genes and gene array technology to explore the range of changes in gene expression. Third, since a recent publications has described neuron loss confined to area 8A, we will use design-based stereology to obtain estimates of the total number of neurons in three adjacent areas of the prefrontal cortex (areas 9, 46 and 8A) and five adjacent areas of the medial temporal lobe (areas 28, 35, TH, TL and TF).And we will adapt stereological sampling to search for regional areas of loss within each of these areas. Fourth, to assess age-related changes in corticocortical pathways of the prefrontal cortex using in vivo neurophysiological methods to measure the compound action potential, diffusion tensor MRI to assess white matter integrity, and immunocytochemistry to assess activated microglia and reactive astrocytes in the same white matter pathways compared with the rest of the forebrain. These studies will be conducted in young adult, middle aged and elderly monkeys that have been behaviorally characterized on tasks assessing frontal lobe and medial temporal lobe function. By examining these issues in monkeys that cover the full adult life span we will be able to determine which changes occur first and their relationship to age-related cognitive decline.